Co-pending U.S. patent application Ser. No. 09/422,895 entitled xe2x80x9cMethods and Apparatus for Measuring Electrical Properties of a Groundxe2x80x9d by Mats Lagmanson is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to the field of geophysical electrical measurements. More particularly, it concerns methods and apparatus for measuring an electrical property of the ground by utilizing a graphite electrode for transmitting a current into the ground and for measuring a potential associated with that current.
2. Description of Related Art
Various geophysical applications utilize electrical surveys to determine a sub-ground resistivity distribution by making electrical measurements on the ground. From such measurements, the resisitivity of the sub-ground may be estimated and related to various geological parameters such as mineral and fluid content, porosity, and water saturation.
Resistivity measurements are typically made by applying current directly into the ground using a pair of transmitting electrodes. Resulting potential differences may then be measured directly using several other receiving electrodes. The receiving electrodes are typically arranged in an array or grid.
Induced polarization relies upon the fact that current injected into the ground causes some materials to become polarized. The phenomenon of polarizing the ground is often referred to as chargeability. The type and amount of chargeability associated with the ground may be measured by taking time domain and/or frequency domain measurements. In general, such measurements determine the degree to which the ground has been polarized by comparing a source signal transmitted to the ground with a measured decay signal after the source signal has been shut off. One or more physical properties of the ground may be correlated with the degree of polarization.
Both induced polarization and resistivity measurements depend upon electrodes for transmitting and receiving electrical information. Conventional electrodes are made from metal. Although these electrodes have proven their usefulness over the years, problems nevertheless remain. One main problem with metal electrodes is that they cannot withstand long-term, maintenance-free use.
In certain applications, it is desirable to position electrodes underground or underwater for an extended period of time so that the ground can be monitored on a routine basis. For instance, if one is monitoring the conditions of a waste site, it may be useful to position electrodes underground and make measurements once a month for several years to determine, for instance, trends in the recorded data. Metal electrodes, however, are not amenable to this type of measurement.
Metal electrodes demonstrate degradation after several uses. In particular, after being subjected to operating conditions while underground or underwater, metal electrodes exhibit corrosion and electrochemical degradation that impairs their effectiveness. To combat these problems, users typically retrieve the electrodes from the ground (or from underwater) so that they may be replaced or cleaned and maintained. The electrochemical degradation may make it necessary to totally replace the electrode after only a short period of use. During the cleaning, the corrosion is removed from the electrode so that it may once again operate properly. The requirement for repeated replacement or cleaning makes metal electrodes difficult and expensive, especially in applications where long-term measurements are desired.
Therefore, it would be advantageous to provide for the ability to measure an electrical property of the ground using an electrode that can withstand, on a long-term basis, operating conditions underground or underwater. In particular, it would be advantageous to have an electrode that could be buried underground or underwater without having to worry about retrieving it for cleaning or replacement. Such an electrode could be used to study long-term trends without undue maintenance.
As used herein, the indefinite articles xe2x80x9caxe2x80x9d and xe2x80x9canxe2x80x9d are to connote xe2x80x9cone or morexe2x80x9d unless otherwise noted. As used herein, by xe2x80x9cground,xe2x80x9d it is meant any surface or sub-surface for which an electrical property may be measured. xe2x80x9cGroundxe2x80x9d may refer to, but is not limited to, the surface of the earth, surface of lakes, rivers, oceans, bottoms of lakes, rivers, oceans, holes, and/or any man-made structures. As used herein, by xe2x80x9celectrical potential,xe2x80x9d it is meant any potential. For example, xe2x80x9celectrical potentialxe2x80x9d applies to, but is not limited to, potentials associated with, for instance, resistivity and/or induced polarization.
In one respect, the invention is an electrode for measuring a property of a ground. The electrode includes a graphite body, a first opening, and a second opening. The first opening extends longitudinally throughout the graphite body for receiving a multi-wire cable. The second opening, within the graphite body, affixes one or more wires of the multi-wire cable to the graphite body.
In other respects, the graphite body may include graphite having a particle size of about 5 microns. The graphite body may include graphite having a shore hardness of about 75. The graphite body may include graphite having an apparent density of about 1.91 grams per cubic centimeter. The graphite body may include graphite having a flexural strength of about 14295 pounds per square inch. The graphite body may include graphite having a resistivity of about 0.000591 ohm-inches. The second opening of the electrode may include a threaded opening for receiving a screw that affixes one or more wires to the graphite body. The electrode may include a waterproof cover upon at least a portion of its graphite body. The waterproof cover may include polyurethane that covers opposite ends of the graphite body.
In another respect, the invention is a method for measuring an electrical property of a ground. A graphite electrode is positioned adjacent the ground. A multi-wire cable is passed through a first opening extending longitudinally throughout the graphite electrode. One or more wires of the multi-wire cable are affixed to the graphite electrode using a second opening within the graphite electrode. A current is delivered to the ground using the graphite electrode, and an electrical potential associated with the current is measured.
In other respects, the electrical property being measured may include resistivity. The electrical property being measured may include induced polarization. The method may also include rendering a map of the ground using the measurement of electrical potential. The graphite electrode may include graphite having a particle size of about 5 microns. The second opening of the electrode may include a threaded opening, and the method may include securing the one or more wires to the graphite electrode with a screw. The positioning of the electrode in the method may include placing the graphite electrode underground or underwater.
In another respect, the invention is a method for long-term, maintenance-free measurement of an electrical property of a ground. A graphite electrode is positioned underground or underwater for more than three months without removing the electrode for replacement or cleaning. A multi-wire cable is passed through a first opening extending longitudinally throughout the graphite electrode. One or more wires of the multi-wire cable are affixed to the graphite electrode with a screw coupled to the graphite electrode. A current is delivered to the ground using the graphite electrode after three months of being underground or underwater without having been replaced or cleaned, and an electrical potential associated with the current is measured.
Generally speaking, the present disclosure describes how electrical resistivity and induced polarization measurements may be performed in order to investigate the underground. At least four graphite electrodes may be implanted in the ground. One pair transmits a signal into the earth and the other electrodes receive the signal. With proper processing known in the art, the induced polarization and resistivity of the earth can be calculated.
Conventional electrodes deteriorate with time depending on corrosion and depending on electrochemical processes when the electric current is passed through the electrodes. For monitoring purposes, it is important to have a functioning monitoring system in the ground for more than 30 years, and perhaps as long as 100 years. The inventor has found that, when using electrodes made of graphite, there is no deterioration caused by corrosion or electro-chemical effect. Thus, such electrodes can be used for a long period of timexe2x80x94perhaps as long as 100 yearsxe2x80x94with little or no maintenance.
This disclosure deals with using electrodes made of graphite to eliminate the damaging effects of corrosion and electrochemical degradation on electrodes. It also deals with implanting graphite electrodes in the ground for resistivity or induced polarization monitoring. It also deals with graphite electrodes being placed in a lake, river or the sea to perform resistivity or induced polarization monitoring. In this regard, the graphite electrodes may be towed in a lake, river or the sea to perform a continuous resistivity or induced polarization survey.